A spark plug has a center electrode and a ground electrode. The center electrode is held by an insulator, and the ground electrode is fixed by a metal shell accommodating that insulator. Between the center electrode and the ground electrode, a clearance for generating a spark discharge is formed. In the followings, this clearance is also referred to as “spark gap.” The spark plug ignites a gas supplied into a combustion chamber of an internal combustion engine by generating the spark discharge at the spark gap.
In some spark plugs, a metallic member is joined to an opening end part that is an end part of an opening edge in the front end side of the metal shell. For example, in the spark plug in JP-UM-A-2-37485, an annular ground electrode is weld-joined to the opening end part in the front end side of the metal shell. In the followings, the metallic member joined to the opening end part in the front end side of the metal shell is also referred to as “front end member.”